1. Field of the Invention
The present invention is related to a data reproducing apparatus for reproducing data by executing a decoding process after a signal read from a recording medium has been equalizing-processed by an adaptive equalizing circuit.
2. Description of the Related Art
Various sorts of recording media such as magnetic tapes, magnetic disks, optical disks, and magneto-optical disks, and recording/reproducing apparatus capable of recording/reproducing these recording media are commercially available.
Generally speaking, in the reproducing apparatus capable of playing these recording media, the signals (RF signals) read from the reading media by the reproducing heads are equalizing-processed by way of analog equalizers and digital filters. After the equalized RF signals are binary-coded, the binary-coded signals are processed by the decoding process and the error correction process to reproduce the data.
To achieve the optimum equalizing characteristic during the reproducing operation, a so-called "adaptive type equalizing circuit" is employed in order that the optimum characteristic is automatically set in response to the input signal, considering fluctuations in the mechanical and electrical characteristics with respect to the respective recording media and recording/reproducing apparatuses. As a result, the error rate can be improved. The adaptive type equalizing circuit is constituted by, for instance, a transversal filter, and an adaptive equalizing coefficient calculating circuit for variably setting a coefficient applied to this transversal filter in response to an input (predicted error predictable from output).
However, the reproducing apparatus employing the adaptive equalizing circuit encounters the following problems.
First, when an abnormal signal is fed into the adaptive equalizing circuit provided in the reproducing apparatus, the adaptive equalizing loop is erroneously operated.
Usually, the abnormal reproducing states will occur with a relatively high probability in the reproducing apparatus. For instance, when a reproducing head is entered into an unrecorded portion on a recording medium such as a magnetic tape, when the reproducing head is returned from an unrecorded portion to a recorded portion thereof, when a large number of dropouts of reproduced information occur, or when a head lock of a magnetic head happens to occur, the RF signal read from the recording medium is brought into the abnormal state. Then, when this abnormal RF signal is entered into the adaptive equalizing circuit, this adaptive equalizing circuit would be diverged.
It should be understood that a "divergence" implies the below-mentioned phenomenon. That is, even when the following operations are repeatedly carried out and thereafter the reproducing signal is returned to the proper state, the operation cannot be returned to the optimum equalizing state. The operations are performed as follows. When the abnormal reproducing signal is inputted, the adaptive equalizing coefficient calculating circuit detects the erroneous predicted error, and then the calculation result of the erroneous predicted errors is stored in the adaptive equalizing coefficient calculating circuit. As a result, the improper tap coefficient is given to the transversal filter.
This adverse phenomenon would give great difficulties when the reproducing apparatus equipped with the adaptive equalizing circuit is realized.
For example, considering such a case that a portable recording medium is used in a DAT (digital audio tape) reproducing apparatus, this reproducing apparatus is required under good compatibility to reproduce various recording media (magnetic tapes etc) which have been recorded by various recording apparatuses. That is, even in such a case that the characteristic of the loaded recording medium, and the characteristic of the recording apparatus for recording this recording medium are greatly deviated from the standard characteristics, the reproducing operation must be carried out under better error rate.
In view of this consideration, when the adaptive equalizing circuit is employed, the equalizing characteristic is optimized, so that the error rate can be largely improved.
However, when such a recording medium is reproduced which owns a characteristic greatly deviated from the standard characteristic, the error rate immediately after the reproducing operation is commenced could not be largely improved. This is because when such a recording medium is reproduced, there is a large deviation between the target equalizing characteristic and the initial equalizing characteristic. As a consequence, the time period required while the adaptive equalizing circuit is converged into the optimum equalizing characteristic state would be prolonged.
This may cause a large drawback in such a system that starting/stopping operations of reproduction are frequently repeated, for example, a magnetic tape is used as a back-up storage of a computer.
The problems owned by the reproducing apparatus capable of reproducing the portable type recording medium have been described. A similar problem may be produced in a computer hard disk drive, namely, fixed recording media. In other words, considering such a case that the reproducing characteristic is deteriorated due to the aging aspect, there is a large deviation between the equalizing characteristic proposed by the adaptive equalizing circuit and the initial equalizing characteristic. Therefore, the time period required when the adaptive equalizing circuit is converged into the optimum equalizing characteristic condition would be prolonged, so that the function capable of improving the error rate could not be effectively realized.
Also, there are many recording media having various different characteristics even in one same sort of recording media acceptable by a reproducing apparatus. For instance, assuming now that the above-described recording media are magnetic tape media, as the presently available magnetic tape media, there are metal single layer coating type magnetic tapes, metal double layer coating type magnetic tapes, and metal vapor deposition type magnetic tapes. Then, the high frequency characteristics of these magnetic tapes are improved in this order. Therefore, considering now that the reproducing apparatus should accept various recording media having the different characteristic belonging to even the same sort of recording media, the reproducing apparatus must employ the coefficient tables different from each other, depending upon the different characteristics of the recording media, as the initial coefficients of the adaptive equalizing circuits. As a consequence, there is another problem that most of the storage regions such as an EEPROM capable of saving these coefficient tables are occupied.